This invention relates to a roll assembly for a fiber drafting machine such as a roving or spinning frame. These machines are used in the processing of staple fibers of either natural or synthetic origin into spun yarn. As processing of the fiber takes place, the strand is greatly increased in length and decreased in size by passing the strand through a plurality of rolls. In a conventional ring spinning frame, for example, the strand is passed between a pair of top rolls which are weighted so that they have a definite grip on the strand. The middle rolls (or aprons) are driven at a slightly greater surface speed than the back rolls, causing fibers within the strand to be longitudinally displaced relative to each other. A pair of front rolls move at a surface speed much greater than either the middle or back rolls, causing the strand to be reduced to a much finer size. Twisting of the strands takes place as drafting occurs so that the finished yarn product has a suitable amount of twist for its end use.
In the past, yarn processing took place at relatively slow speeds. However, in recent years the rapid increase in the cost of labor and the simultaneous increase in the speed and sophistication of roving and spinning frames has greatly increased the mechanical stress on the drafting rolls.
Typically roller bearings, such as ball or needle bearings, have previously been used in spinning and roving frame drafting rolls. When operated at relatively low speeds and under relatively low stress conditions, ball bearings perform adequately with relatively infrequent lubrication. However, greater speed and operating stress significantly increase the frequency of bearing failure and, in addition, substantially increase the frequency with which routine maintenance such as cleaning and lubrication must be performed. Even at modern, high production rates, yarn production from a single position on a spinning frame is relatively low. For this reason, the typical yarn mill may have several hundred frames, each having a hundred or more positions. Since each position has several sets of rolls, each of which have two roll assemblies, the number of bearings which must be regularly lubricated and cleaned is significant. The amount of labor required to perform these routine tasks can detract significantly from the efficiencies received from higher production rates.
In addition to the labor involved in lubricating bearings, the presence of a lubricant itself is inherently undesireable, since the lubricant, if it leaks from the bearing, can soil the yarn being processed, and, within bearing, can attract lint which interferes with bearing performance.
Low friction materials such as tetrafluoroethylene and polyimide materials have previously been used for making seals, gaskets, valve seats, insulators, bearings, bushings, piston rings, washers, wear strips and similar items. This type of product is commercially available from E. I. du Pont de Nemours and Company as VESPEL.RTM. polyimide and from Rogers Corporation as INVEX.RTM. polyimide. Structures can be prepared from such materials by either compression molding the resin powder, or machining preformed shapes on standard metal-working equipment in much the same manner as brass. Products such as polyimide resin are characterized by a low natural coefficient of friction without secondary lubrication, good wear resistance, continuous operation at temperature ranges between cryogenic to over 500.degree. F. (288.degree. C.).